Molecular weight distribution (MWD), or polydispersity, is a well known variable in polymers. The molecular weight distribution, sometimes described as the ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) (i.e., M.sub.w /M.sub.n) can be measured directly, e.g., by gel permeation chromatography techniques, or more routinely, by measuring I.sub.10 /I.sub.2 ratio, as described in ASTM D-1238. For linear polyolefins, especially linear polyethylene, it is well known that as M.sub.w /M.sub.n increases, I.sub.10 /I.sub.2 also increases.
John Dealy in "Melt Rheology and Its Role in Plastics Processing" (Van Nostrand Reinhold, 1990) page 597 discloses that ASTM D-1238 is employed with different loads in order to obtain an estimate of the shear rate dependence of melt viscosity, which is sensitive to weight average molecular weight (M.sub.w) and number average molecular weight (M.sub.n).
Bersted in Journal of Applied Polymer Science Vol. 19, page 2167-2177 (1975) theorized the relationship between molecular weight distribution and steady shear melt viscosity for linear polymer systems. He also showed that the broader MWD material exhibits a higher shear rate or shear stress dependency.
Ramamurthy in Journal of Rheology, 30(2), 337-357 (1986), and Moynihan, Baird and Ramanathan in Journal of Non-Newtonian Fluid Mechanics, 36, 255-263 (1990), both disclose that the onset of sharkskin (i.e., surface melt fracture) for linear low density polyethylene (LLDPE) occurs at an apparent shear stress of 1-1.4.times.10.sup.6 dyne/cm.sup.2, which was observed to be coincident with the change in slope of the flow curve. Ramamurthy also discloses that the onset of surface melt fracture or of gross melt fracture for high pressure low density polyethylene (HP-LDPE) occurs at an apparent shear stress of about 0.13 MPa (1.3.times.10.sup.6 dyne/cm). Ramamurthy also discloses that "the corresponding shear stresses (0.14 and 0.43 MPa) for linear polyethylenes are widely separated." However, these LLDPE resins are linear resins, and are believed to be those made by Union Carbide in their UNIPOL process (which uses conventional Ziegler-Natta catalysis which results in a heterogeneous comonomer distribution). The LLDPE is reported in Tables I and II to have a broad M.sub.w /M.sub.n of 3.9. The melt fracture tests conducted by Ramamurthy were in the temperature range of 190 to 220 C. Furthermore, Ramamurthy reports that the onset of both surface and gross melt fracture (for LLDPE resins) are ". . . essentially independent of MI (or molecular weight), melt temperature, die diameter (0.5-2.5 mm), die length/diameter ratio (2-20), and the die entry angle (included angle: 60-180 degrees)."
Kalika and Denn in Journal of Rheology, 31, 815-834 (1987) confirmed the surface defects or sharkskin phenomena for LLDPE, but the results of their work determined a critical shear stress at onset of surface melt fracture of 0.26 MPa, significantly higher than that found by Ramamurthy and Moynihan et al. Kalika and Denn also report that the onset of gross melt fracture occurs at 0.43 MPa which is consistent with that reported by Ramamurthy. The LLDPE resin tested by Kalika and Denn was an antioxidant-modified (of unknown type) UNIPOL LLDPE having a broad M.sub.w /M.sub.n of 3.9. Kalika and Denn performed their melt fracture tests at 215 C. However, Kalika and Denn seemingly differ with Ramamurthy in the effects of their L/D of the rheometer capillary. Kalika and Denn tested their LLDPE at L/D's of 33.2, 66.2, 100.1, and 133.1 (see Table 1 and FIGS. 5 and 6).
International Patent Application (Publication No. WO 90/03414) published Apr. 5, 1990 to Exxon Chemical Company, discloses linear ethylene interpolymer blends with narrow molecular weight distribution and narrow short chain branching distributions (SCBDs). The melt processibility of the interpolymer blends is controlled by blending different molecular weight interpolymers having different narrow molecular weight distributions and different SCBDs.
Exxon Chemical Company, in the Preprints of Polyolefins VII International Conference, page 45-66, February 24-27 1991, disclose that the narrow molecular weight distribution (NMWD) resins produced by their EXXPOL.TM. technology have higher melt viscosity and lower melt strength than conventional Ziegler resins at the same melt index. In a recent publication, Exxon Chemical Company has also taught that NMWD polymers made using a single site catalyst create the potential for melt fracture ("New Specialty Linear Polymers (SLP) For Power Cables," by Monica Hendewerk and Lawrence Spenadel, presented at IEEE meeting in Dallas, Tex., September, 1991). In a similar vein, in "A New Family of Linear Ethylene Polymers Provides Enhanced Sealing Performance" by Dirk G. F. Van der Sanden and Richard W. Halle, (February 1992 Tappi Journal), Exxon Chemical Company has also taught that the molecular weight distribution of a polymer is described by the polymers melt index ratio (i.e., I.sub.10 /I.sub.2) and that their new narrow molecular weight distribution polymers made using a single site catalyst are "linear backbone resins containing no functional or long chain branches."
U.S. Pat. No. 5,218,071 (Canadian patent application 2,008,315-A) to Mitsui Petrochemical Industries, Ltd., teaches ethylene copolymers composed of structural units (a) derived from ethylene and structural units (b) derived from alpha-olefins of 3-20 carbons atoms, said ethylene copolymers having [A] a density of 0.85-0.92 g/cm.sup.3, [B] an intrinsic viscosity as measured in decalin at 135 C of 0.1-10 dl/g, [C] a ratio (M.sub.w /M.sub.n) of a weight average molecular weight (M.sub.w) to a number average molecular weight (M.sub.n) as measured by GPC of 1.2-4, and [D] a ratio (MFR.sub.10 /MFR.sub.2) of MFR.sub.10 under a load of 10 kg to MFR.sub.2 under a load of 2.16 kg at 190 C of 8-50, and being narrow in molecular weight distribution and excellent in flowability. However, the ethylene copolymers of U.S. Pat. No. '071 are made with a catalysis system composed of methylaluminoxane and ethylenebis(indenyl)hafnium dichloride (derived from HfCl.sub.4 containing 0.78% by weight of zirconium atoms as contaminates). It is well known that mixed metal atom catalyst species (such as hafnium and zirconium in U.S. Pat. No. '071) polymerizes copolymer blends, which are evidence by multiple melting peaks. Such copolymer blends therefore are not homogeneous in terms of their branching distribution.
WO 85/04664 to BP Chemicals Ltd. teaches a process for the thermo-mechanical treatment of copolymers of ethylene and higher alpha-olefins of the linear low density polyethylene type with at least one or more organic peroxides to produce copolymers that are particularly well suited for extrusion or blow-molding into hollow bodies, sheathing, and the like. These treated copolymers show an increased flow parameter (I.sub.21 /I.sub.2) without significantly increasing the M.sub.w /M.sub.n. However, the novel polymers of the present invention have long chained branching and obtained this desirable result without the need of a peroxide treatment.
U.S. Pat. No. 5,096,867 discloses various ethylene polymers made using a single site catalyst in combinations with methyl aluminoxane. These polymers, in particular Example 47, have extremely high levels of aluminum resulting from catalyst residue. When these aluminum residues are removed from the polymer, the polymer exhibits gross melt fracture at a critical shear stress of less than 4.times.10.sup.6 dyne/cm.sup.2.
All of the foregoing patents, applications, and articles are herein incorporated by reference.
Previously known narrow molecular weight distribution linear polymers disadvantageously possessed low shear sensitivity or low I.sub.10 /I.sub.2 value, which limits the extrudability of such polymers. Additionally, such polymers possessed low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced melt fracture surface properties at relatively low extrusion rates thereby processing unacceptably.